1. Field of the Invention
The present invention relates to a fuel cell stack which includes a stack body formed by stacking a plurality of unit cells in a horizontal direction, and a pair of end plates sandwiching the stack body. Each of the unit cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane (electrolyte) comprising a polymer ion exchange membrane. The electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. The membrane electrode assembly is sandwiched between separators to form a fuel cell. In use, normally, a predetermined number of (e.g., several tens to several hundreds of) fuel cells are stacked together to form a fuel cell stack to obtain the desired electrical energy.
At the time of power generation in the fuel cell, by electrochemical reactions of hydrogen and oxygen, water is produced. Therefore, the power generation performance tends to be changed easily depending on the internal state of the produced water. Therefore, the state of the produced water needs to be managed suitably.
In this regard, for example, a fuel cell apparatus as disclosed in Japanese Laid-Open Patent Publication No. 2001-319673 is known. In the conventional technique, as shown in FIG. 16, a fuel cell stack 3 and a compression stress regulator mechanism 4 are provided. Hydrogen from a hydrogen supply apparatus 1 and oxygen from an oxygen supply apparatus 2 are used as fuels for power generation in the fuel cell stack 3. The compression stress regulator mechanism 4 regulates compression stress applied to the fuel cell stack 3.
The compression stress regulator mechanism 4 includes a surface pressure applying member 5, a spherical body 6, a screw 7, and a motor 8. The surface pressure applying member 5 is attached to an end of the fuel cell stack 3. The surface pressure applying member 5 applies a surface pressure to the fuel cell stack 3. The spherical body 6 applies an axial force uniformly to the surface pressure applying member 5. The screw 7 applies the axial force to the spherical body 6. The motor 8 rotates the screw 7.
According to the disclosure, by operation of the compression stress regulator mechanism 4, compression stress is regulated to adjust the space for movement of water in the fuel cell stack 3 to achieve the desired humidification state in the fuel cell stack 3.
In the fuel cell stack 3, swelling of the electrolyte membrane occurs by the water produced in the power generation. In particular, swelled portion becomes large, in particular, on the lower side in the direction of gravity. Thus, difference in swelling occurs in the electrolyte membrane along the direction of gravity.
However, in the conventional technique, the spherical body 6 presses substantially the center of the surface pressure applying member 5 attached to the end of the fuel cell stack 3, and the swelling difference in the direction of gravity, in the electrolyte membrane cannot be eliminated. Thus, for example, when the fuel cell stack 3 is placed in a casing (box), the load is applied non-uniformly to the fuel cell stack 3 due to the difference in swelling. As a result, the casing is deformed undesirably.